Stimulation fluids are used to create additional permeability in oil and gas reservoirs. This is achieved by pumping the fluid (hydrocarbon or water) down hole. While this is happening three chemicals are mixed at the surface and blended into the fluid to create viscosity, also known as a gel state. The three main chemicals contained in the used fracturing fluid are, in general, as described below. The gelling agent—typically a phosphate ester which is slightly acidic, the activator—typically an iron or aluminum complex which may be a Lewis acid, and the breaker—typically a basic metal oxide.
The viscous fluid is pumped at high rates down hole and is forced into the hydrocarbon reservoir under high pressures. Once the fluid pressure is greater than the rock pressure, the fluid creates a fracture in the rock and flows into the fracture. As this is happening, proppant, usually sand, is then added and pumped downhole to stabilize the fracture and provide porosity.
As the fracture stimulation ends, the breaker chemical in the fluid begins to degrade (or break) the gel, which brings the viscosity down to pre-gel levels. Once the hydrocarbon fluid is broken, the fluid (termed flow-back) is then brought to surface and subsequently sent to oil recyclers for clean-up and sold back into the crude oil system.
Recycling of hydrocarbon fracturing fluids has been limited. Typically a company will reuse the fluid and increase the concentration of all the chemicals used to create and break the gel. This can occur only a limited number of times before the fluid becomes too unstable to create a usable gel system. Previous industrial efforts have used lime to remove residual gels at surface that have not broken. Heating and filtering have also been used to clean the hydrocarbon flow back and remove any solid particles such as sand and clay.